Rotor hub vibration attenuator

ABSTRACT

A vibration attenuator for an aircraft has at least one weight mounted in a rotating system of a rotor hub of the aircraft, each weight being rotatable about an axis of rotation of the hub relative to the hub and to each other weight. Drive means are provided for rotating each weight about the axis of rotation at a selected speed for creating oscillatory shear forces that oppose and attenuate rotor-induced vibrations having a selected frequency. A vertically oriented vibration attenuator is configured to oppose and attenuate vertical rotor induced oscillatory forces that would otherwise travel vertical down the rotor mast and into the airframe.

BACKGROUND

1. Field of the Present Description

The technical field is vibration attenuators for rotor hubs.

2. Description of Related Art

Rotary-wing aircraft, such as helicopters and tiltrotors, have at leastone rotor for providing lift and propulsion forces, and these rotorshave at least two airfoil blades connected to a rotatable hub. Theblades cause vibrations that are a function of the rotational speed ofthe rotor, and aircraft designers have difficulty accurately predictingthe exact vibration modes that a particular rotor configuration willencounter. The vibrations can be transmitted through the rotor mast,through associated powertrain components, and into the airframe of theaircraft. The vibrations can reduce the life of affected components andcause undesirable vibrations for passengers. Various types of vibrationattenuation systems have been developed to reduce or eliminate thesevibrations. The prior art includes airframe-mounted vibrationattenuators and at least one mast-mounted system.

Active systems in the prior art act at a specific point on the airframeto reduce vibrations, and this can result in amplified vibrations inother locations on the airframe. However, a passive mast-mountedrotating balancer for vibration reduction was disclosed in U.S. Pat. No.3,219,120 and in an American Helicopter Society paper entitled, “UREKA-AVibration Balancing Device for Helicopters” (January 1966, Vol. 11, No.1). The UREKA (Universal Rotor Excitation Kinematic Absorber) deviceuses heavy rollers which revolve in a circular steel track to create anoscillatory force to minimize vibration. The rollers are free to rotateand position themselves relative to the position of the rotor, and therollers will automatically achieve the correct position to minimizevibration if the mast attachment point possesses specific dynamiccharacteristics. However, the UREKA system only prevents an imbalance ofthe rotor, and does not oppose other rotor-induced vibrations. Thedynamic characteristics necessary for proper operation are basicallythose of a supercritical shaft. If the mast attachment point does notpossess these characteristics, then the UREKA device will amplifyvibration rather that attenuate it. In addition, since the position ofthe rollers is governed by the motion of the mast attachment point, thedevice is susceptible to gusts and other transients which may disturbthe roller position, creating a vibration transient.

For application to tiltrotors, where large changes in gross weight androtor rotational speed are present, the UREKA device may not functionproperly, as the dynamic characteristics of the mast attachment pointwould vary considerably. The V-22 tiltrotor, for example, has dynamiccharacteristics which prevent the use of the UREKA design. One methoddeveloped for the 3-blade V-22 aircraft includes passive pendulums forcontrolling vibrations.

Although great strides have been made in the art of vibrationattenuators for rotor hubs, significant shortcomings remain.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the present system are setforth in the appended claims. However, the system itself, as well as apreferred mode of use, and further objectives and advantages thereof,will best be understood by reference to the following detaileddescription when read in conjunction with the accompanying drawings,wherein:

FIG. 1 is an oblique view of an aircraft having a vibration attenuationsystem;

FIG. 2 is an oblique, partially sectioned view of a proprotor of theaircraft of FIG. 1;

FIG. 3A is a schematic view of a portion of the vibration attenuationsystem of the aircraft of FIG. 1;

FIG. 3B is a schematic view of a portion of the vibration attenuationsystem of the aircraft of FIG. 1;

FIG. 4A is a schematic view of a portion of the vibration attenuationsystem of the aircraft of FIG. 1;

FIG. 4B is a schematic view of a portion of the vibration attenuationsystem of the aircraft of FIG. 1;

FIG. 5A is a schematic view of a portion of the vibration attenuationsystem of the aircraft of FIG. 1;

FIG. 5B is a schematic view of a portion of the vibration attenuationsystem of the aircraft of FIG. 1;

FIG. 6 is an oblique, partially sectioned view of a proprotor having analternative embodiment of a vibration attenuation system;

FIG. 7 is an oblique, partially sectioned view of a proprotor having analternative embodiment of a vibration attenuation system;

FIG. 8 is an oblique, partially sectioned view of the proprotor of FIG.7;

FIG. 9 is an oblique, partially sectioned view of a proprotor having analternative embodiment of a vibration attenuation system;

FIG. 10 is an oblique, partially sectioned view of the proprotor of FIG.9;

FIG. 11 is a side, partially sectioned view of a proprotor having analternative embodiment of a vibration attenuation system;

FIG. 12 is an oblique, partially sectioned view of a proprotor having analternative embodiment of a vibration attenuation system;

FIG. 13 is a side, partially sectioned view of a proprotor having analternative embodiment of a vibration attenuation system; and

FIG. 14 is a side, partially sectioned view of a proprotor having analternative embodiment of a vibration attenuation system.

DETAILED DESCRIPTION

A vibration attenuator system for a rotor hub provides for vibrationattenuation in a rotary-wing aircraft by reducing the magnitude of rotorinduced vibratory forces acting on the airframe. The vibrationattenuator system includes vibration attenuators attached to a rotormast in the rotating system of the rotor hub for rotation about the mastaxis in the same or opposite direction as the mast. Vibratory shearforce is generated by rotating pairs of unbalanced weights at high speedto create large centrifugal forces, and the weights may be driven byelectric motors or by torque provided by the mast. The rotational speedof the weights will typically be a multiple of the mast rotational speedto create shear forces for canceling rotor induced vibrations, which canbe rotating in the same direction as the proprotor or in the oppositedirection. The amplitude of the shear force is controlled by indexingthe positions of the weights of each pair relative to each other as theyrotate about the axis of the mast, while the phase of the shear force isadjusted by equally phasing each pair of weights relative to the rotor.A microprocessor-based control system uses feedback from vibrationsensors to command the operation of the vibration attenuators so as tominimize vibrations transmitted to the airframe.

This system is an improvement over methods now being used because it islighter weight, more compact, and is capable of better vibrationreduction. The principal advantage of this device is that it cancels thesource of vibratory loads, thereby reducing vibration throughout theentire aircraft. As described above, competing active systems act toreduce vibrations at a specific point in the airframe, which can causeamplified vibrations at other locations in the airframe. By reducing themagnitude of rotor-induced vibratory loads, the vibration attenuatorsystem can improve the fatigue life of critical structural components,reduce vibration of avionics, reduce engine vibration, and improvepassenger comfort.

FIG. 1 is an oblique view of a rotary-wing aircraft having a vibrationattenuator system, which is described below. Aircraft 11 is arotary-wing aircraft, specifically a tiltrotor aircraft, having afuselage 13 and wings 15 extending from fuselage 13. Fuselage 13 andwings 15 comprise the airframe of aircraft 11. A rotatable nacelle 17 islocated at the outer end of each wing 15 for housing an engine (notshown), and each engine is configured for providing torque to causerotation of an attached proprotor 19. Each proprotor 19 has a pluralityof blades 21, which are connected to a hub (see FIG. 2) located beneathan aerodynamic fairing, referred to as a spinner 23.

FIG. 2 is an oblique view of a proprotor 19 with blades 21 removed fromyoke 25 of the hub. Holes 27 are formed in spinner 23 (a portion iscutaway for ease of viewing) for allowing portions of yoke 25 toprotrude for attachment of blades 21. A mast 29 is connected to anoutput of the engine for transfer of torque from the engine to mast 29.In the configuration shown, a constant-velocity drive assembly 31 issplined to mast 29 for rotation with mast 29, and yoke 25 is connectedto drive assembly 31. Drive assembly 31 allows for yoke 25 to gimbalrelative to mast 29 as mast 29 drives yoke 25 in rotation about mastaxis 33.

In the configuration shown, two vibration attenuators 35, 37 are carriedon an end portion of mast 29. Attenuators 35, 37 operate in asubstantially identical manner and have similar construction, with eachhaving a rotatable weight, such as weighted disk 39, and an electricmotor 41. Motors 41 are splined or otherwise affixed to mast 29 forrotation with mast 29, and each motor 41 is preferably a brushlessstepper motor configured for driving the associated disk 39 in rotationabout mast axis 33 in a selected direction and at a selected rotationalspeed relative to mast 29. Each disk 39 has a center of mass that islocated a radial distance from mast axis 33, such that rotation of eachdisk 39 causes an oscillatory, radially outward shear force on mast 29in the plane of rotation. While shown as having a disk-shapedconstruction, weights of attenuators 35, 37 may be of other types, suchas elongated arms. By using a stepper-type motor 41, each disk 39 can berotated to a selected angle, or indexed, relative to the other disk 39during their rotation at the same speed and direction about mast axis33. In addition, disks 39 may be commanded to rotate together at thesame speed and direction and at a selected phasing relative to proprotor19 while maintaining the same index setting.

Referring also to FIG. 1, a microprocessor-based control system 43 isshown as being located in fuselage 13 and is configured to automaticallycontrol the operation of vibration attenuators 35, 37. Control system 43preferably comprises feedback sensors, such as sensors 45 located onfuselage 13 and wings 15, to provide vibration feedback data. Thoughshown in particular locations, sensors 45 may be installed in otherlocations, such as within nacelles 17. Use of sensors 45 allows controlsystem 43 to control the operation of vibration attenuators 35, 37 basedon measurements of vibrations transmitted into and through the airframe.Control system 43 may alternatively control operation of vibrationattenuators 35, 37 based on other data, such as airspeed, rotor speed,blade pitch angle, nacelle angle, amount of rotor thrust, and/or similarparameters.

Operational control preferably includes commanding at least rotationalspeed, rotational direction, indexing of pairs of disks 39, and phasingof pairs of disks 39. Control system 43 and/or vibration attenuators 35,37 may be provided with “fail-off” features to prevent vibrationattenuators 35, 37 from inducing unintended and undesirable vibrationsin the event of failure of one or more components of the vibrationattenuation system. Inputs to control system may include aircraft grossweight, load factor, altitude, airspeed, and rpm. In addition, controlsystem 43 may be optimized for use on tiltrotor aircraft 11 by alsobasing commands on the angle of nacelles 17 and other tiltrotor-specificparameters. Use of control system 43 to control vibration attenuators35, 37 means that attenuators 35, 37 are less susceptible to transients,such as gusts, than the prior-art UREKA system and is not dependant onthe dynamic characteristics of the mast.

In operation, control system 43 independently commands each motor 41 todrive associated disk 39 in the selected rotational direction and at theselected rotational speed. For example, disks 39 may be driven in thesame rotational direction as mast 29 and at a multiple of the rotationalspeed of mast 29. Disks 39 are unbalanced, and they create oscillatoryshear forces in the plane of rotation at a frequency described as thenumber of cycles per revolution of proprotor 19 (n/rev). When the shearforces are equal in amplitude to the aerodynamic n/rev forces ofproprotor 19 and opposite their phase, then no vibratory force will betransmitted to the airframe. For example, if a four-blade proprotor 19is rotating at 400 revolutions per minute, and the vibration attenuatorsare to oppose 4/rev vibrations by rotating in the direction of proprotor19, motors 41 will cause disks 39 to spin at 4× the speed of proprotor19 relative to the airframe. Because mast 29 is spinning in the samedirection as disks 39 relative to the airframe at 1× the speed ofproprotor 19, disks 39 will be spinning at 3× the speed of proprotor 19relative to mast 29 and proprotor 19. Likewise, if disks 39 are tooppose 8/rev vibrations by rotating in the opposite rotation ofproprotor 19, motors 41 will cause disks 39 to spin at 8× the speed ofproprotor 19 relative to the airframe. Because mast 29 is spinning inthe opposite direction at 1× the speed of proprotor 19, the disks willbe spinning at 9× the speed of proprotor 19 relative to mast 29 andproprotor 19.

The magnitude of the oscillatory shear force is determined by therelative position of the center of mass of disks 39. FIGS. 3A and 3B, 4Aand 4B, and 5A and 5B illustrate the relative rotational positions ofdisks 39 of vibration attenuators 35, 37 for three modes of operation,with each A and B figure showing one of disks 39 as viewed along mastaxis 33. In each figure, the direction of rotation of mast 29 is shownby arrow 47, and the direction of rotation of disk 39 is shown by arrow49.

As described above, each disk 39 has a center of mass located a radialdistance from mast axis 33, and this may be accomplished, for example,by locating a mass 51 along a peripheral portion of each disk 39. Mass51 may be formed as an integral portion of disk 39 or may be formed as aseparate component and attached to disk 39. To provide for additionaltuning of attenuators 35, 37, each mass 51 may be configured to bereplaceable, for example, by a similarly constructed mass 51 having moreor less mass. Mass 51 may also be constructed of multiple pieces,allowing mass 51 to be adjusted by removing or adding pieces. Thoughshown as having only one mass 51, it should be understood that disks 39may configured to have more than mass 51.

If masses 51 of vibration attenuators 35, 37 are diametrically opposed,as shown in FIGS. 3A and 3B, while disks 39 are driven in rotation atthe same speed, then the amplitude of the vibratory force will be zero.This is due to the fact that each disk 39 causes an equal and oppositeshear force that cancels the force caused by the other of disks 39. Ifdisks 39 are indexed during rotation so that masses 51 are aligned, asshown in FIGS. 4A and 4B, the shear force is the maximum magnitude thatvibration attenuators 35, 37 can produce for any given rotational speed.Any magnitude between zero and the maximum is available by changing therelative angle of disks 39, and FIGS. 5A and 5B show disks 39 as havingbeen indexed relative to each other at an angle of approximately 45degrees.

Proprotor 19 is described as having only one pair of vibrationattenuators 35, 37, though additional pairs of attenuators may be addedto oppose additional vibration modes (8/rev, 12/rev, etc.). Additionalattenuators are added in a coaxial arrangement along mast axis 33, andeach pair may comprise weights having a different weight than disks 39and operating at a selected rotational speed different than disks 39. Itshould be noted that the attenuators will be different for differenttypes of rotors, as the weights will be optimized for the particularapplication.

FIG. 6 illustrates a portion of an alternative embodiment of a proprotor53, which is constructed similarly to proprotor 19 of FIGS. 1 and 2.Proprotor 53 has a yoke 25 attached to a drive assembly 31, and driveassembly 31 transfers torque from mast 29 to yoke 25. A spinner 23 (aportion is cutaway for ease of viewing) is installed as an aerodynamicfairing for the hub of proprotor 53. Proprotor 53 differs from proprotor19, in that proprotor 53 has two vibration attenuators 55, 57, which arecoaxially arranged on mast axis 33. Each attenuator 55, 57 has a pair ofweighted disks 59, 61 and a pair of stepper motors 63 (only one of eachattenuator 55, 57 being visible in the view of FIG. 6). Each attenuator55, 57 rotates the associated disks 59, 61 in the same direction and atthe same rotational speed, though disks 59, 61 of the other attenuator55, 57 preferably rotate at a different speed and may rotate in adifferent direction. A control system, such as control system 43 of FIG.1, is preferably provided for controlling the operation of both pairs ofdisks 59, 61, including indexing and phasing of the disks in each pair,as described above for disks 39, 41. In operation, having twoattenuators 55, 57 allows for both attenuators 55, 57 to suppressvibrations simultaneously. Also, having two attenuators 55, 57 allowsfor only one attenuator 55, 57 to suppress a selected vibration whilethe other attenuator 55, 57 is indexed to produce no net shear force.

FIGS. 7 and 8 illustrate a portion of an alternative embodiment of aproprotor 65, which is constructed similarly to proprotor 19 of FIGS. 1and 2. Proprotor 65 has a yoke 25 attached to a drive assembly 31, anddrive assembly 31 transfers torque from mast 29 to yoke 25 for rotationof proprotor 65 about mast axis 33. A spinner 23 (a portion is cutawayfor ease of viewing) is installed as an aerodynamic fairing for the hubof proprotor 65. Proprotor 65 has a vibration attenuator 67, comprisingan adjustable weight assembly 69, which is configured to be driven inrotation relative to mast 29 and about mast axis 33 by stepper motor 71.Weight assembly 69 has at least one weight 73 that is movably attachedto weight support 75 for positioning along track 77 during operation ofproprotor 65. This configuration allows for weight 73 to be selectivelymoved to any position between an inner radial position, which providesfor minimal or no shear forces as weight assembly 69 spins, and an outerposition, which provides for maximum shear forces. FIG. 7 showsproprotor 65 with weight 73 having been moved to an inner position, andFIG. 8 shows proprotor 65 with weight 73 having been moved to anintermediate position. A control system, such as control system 43 ofFIG. 1, is preferably provided for controlling the parameters ofoperation of vibration attenuator 67, including positioning of weight73, speed of rotation, direction of rotation, and phasing of the shearforces relative to the position of the rotor.

In operation, control system 43 commands motor 71 of vibrationattenuator 67 to rotate weight assembly 69 at a selected rotationalspeed and direction relative to mast 29, and control system 43 alsocommands weight 73 to move to a selected position along track 77 forproducing a selected amount of shear force. In addition, control system43 will command motor 71 to rotate weight assembly 69 in a manner thatproduces a selected phasing of the shear forces relative to proprotor65.

FIGS. 9 and 10 illustrate a portion of an alternative embodiment of aproprotor 79, which is constructed similarly to proprotor 65 of FIGS. 7and 8. Proprotor 79 has a yoke 25 attached to a drive assembly 31, anddrive assembly 31 transfers torque from mast 29 to yoke 25. A spinner 23(a portion is cutaway for ease of viewing) is installed as anaerodynamic fairing for the hub of proprotor 79. Proprotor 79 differsfrom proprotor 65, in that proprotor 79 has two vibration attenuators81, 83, which are coaxially arranged on mast axis 33. Each attenuator81, 83 has a rotatable weight assembly 85 and a stepper motor 87, andeach weight assembly 85 comprises at least one weight 89 movablyattached to weight support 91 for selective positioning along track 93during operation of proprotor 79. Motor 87 of each attenuator 81, 83rotates the associated weight assembly 85 at a selected rotational speedand direction, and weight assemblies 85 may rotate in the same oropposite directions and at similar or varying speeds. A control system,such as control system 43 of FIG. 1, is preferably provided forcontrolling the operation of both vibration attenuators 81, 83,including phasing of weight assemblies 85 relative to proprotor 79. FIG.9 shows proprotor 79 with weight 89 of attenuator assembly 81 havingbeen moved to an outer position, whereas weight 89 of attenuator 83 isshown having been moved to an inner position. FIG. 10 shows both weights89 having been moved to outer positions.

Vibration attenuators 81, 83 are shown as having weights adjustable fordistance from axis 33, allowing for each attenuator 81, 83 to be usedfor attenuating a specific vibration. However, another embodiment of aproprotor includes the use of similar attenuators, in which each weightis positioned or formed on an elongated weight support in a selectedfixed position. This type of configuration requires the use of twoattenuators to attenuate a specific vibration, and they are controlledin a manner like that for vibration attenuators 35, 37.

Other embodiments of the vibration attenuators described above mayinclude a gear-type drive system for driving the weights in rotationrather than using electric motors. This type of attenuator would operatewithout requiring a large external source of power, as the powerrequired for operation is preferably taken from the mast. A smallelectric current may be used for electric motors to position the indexedweights about the mast axis for phasing, but once phased, the parasiticpower requirement is negligible and is derived from the mast torque.

Another feature that may be incorporated in the vibration attenuatorsdescribed above is a “standpipe” configuration for mounting of theattenuators. FIG. 11 shows an example embodiment, in which a mast 95encloses a coaxial standpipe 97. In FIG. 11, mast 95 is show with aportion removed for ease of viewing standpipe 97. Mast 95 rotatesrelative to the airframe (not shown) about axis 99 for rotating anattached proprotor (not shown). Standpipe 97 is stationary relative tothe airframe, and bearings 101 are located between an outer surface ofstandpipe 97 and an inner surface of mast 95 to allow for the relativemotion of mast 95 relative to standpipe 97. In the embodiment shown, twoattenuators 103, 105 each comprise a motor 107 and a weighted disk 109.Attenuators 103, 105 are mounted to a narrowed section 111 at an outerend of standpipe 97. An optional platform 113 may be provided onstandpipe 97 for mounting attenuators 103, 105 or other embodiments ofthe attenuators described above. In operation, motors 107 rotate disks109 attenuators 103, 105 in a similar manned as those described above,allowing attenuators 103, 105 to produce oscillatory shear forces onstandpipe 97. These shear forces are then transferred into mast 95through bearings 101. It should be noted that more or fewer attenuatorsthan is shown may be mounted on standpipe 97. It should also be notedthat a standpipe configuration is particularly useful with the gear-typedrive system described above.

FIG. 12 illustrates a portion of an embodiment of a proprotor 1209,which is constructed similarly to proprotor 19 of FIGS. 1 and 2, exceptfor the addition of attenuator 1201 to treat vibration along axis 33 ofmast 29. Proprotor 1209 has a yoke 25 attached to a drive assembly 31,and drive assembly 31 transfers torque from mast 29 to yoke 25. Aspinner (shown in FIGS. 1 and 2) is installed as an aerodynamic fairingfor the hub of proprotor 1209. Proprotor 1209 differs from proprotor 19,in that proprotor 1209 has a vibration attenuator 1201, which is coupledto standpipe 97 (shown in FIG. 13), and vertically aligned on mast axis33 in order to selectively attenuate vibration axially along axis 33 ofmast 29. As such, attenuator 1201 operates similar to attenuators 35,37, except for being oriented vertically in line with axis 33 of mast29. Therefore, the discussion herein regarding attenuators 35, 37 isequally applicable to the attenuator 1201, except for the attenuator1201 oriented and configured to treat vertical vibrations instead of therotor plane forces that attenuators 35, 37 are configured to cancel.Attenuator 1201 has a weighted disk 1205 and a stepper motor 1203.Attenuator 1201 may rotate the associated disk 1205 in either rotationaldirection. A control system, such as control system 43 of FIG. 1, ispreferably provided for controlling the operation of disk 1205,including indexing and phasing of the disk, as described above for disks39, 41. It should be appreciated that even though only one attenuator1201 is shown, a plurality of attenuators 1201 may be used. Inoperation, having a plurality of attenuators 1201 allows for suppressionof multiply vibrations simultaneously.

Referring to FIG. 13, one embodiment of proprotor 1301 includesattenuators 103, 105 mounted to standpipe 97, in addition to attenuator1201 (also shown in FIG. 12) mounted to standpipe 97 via a support 1207.Mast 95 rotates relative to the airframe (not shown) about axis 99 forrotating an attached proprotor (not shown). Standpipe 97 is stationaryrelative to the airframe, and bearings 101 are located between an outersurface of standpipe 97 and an inner surface of mast 95 to allow for therelative motion of mast 95 relative to standpipe 97. In the embodimentshown, two attenuators 103, 105 each comprise a motor 107 and a weighteddisk 109. Attenuators 103, 105 are mounted to a narrowed section 111 atan outer end of standpipe 97. An optional platform 113 may be providedon standpipe 97 for mounting attenuators 103, 105 or other embodimentsof the attenuators described above. In operation, motors 107 rotatedisks 109 attenuators 103, 105 in a similar manned as those describedherein regarding attenuators 35 and 37, such that attenuators 103, 105to produce oscillatory shear forces on standpipe 97. These shear forcesare then transferred into mast 95 through bearings 101. It should benoted that more or fewer attenuators than is shown may be mounted onstandpipe 97. It should also be noted that a standpipe configuration isparticularly useful with the gear-type drive system described above.

FIG. 14 illustrates another embodiment of a proprotor 1413, which isconstructed similarly to proprotor 65 of FIGS. 7 and 8, with theaddition of attenuator 1401. As such, the discussion herein regardingattenuators 103 and 105 is equally applicable to proprotor 1413.Proprotor 1413 has a yoke 25 attached to a drive assembly 31, and driveassembly 31 transfers torque from mast 29 to yoke 25 for rotation ofproprotor 65 about mast axis 33. A spinner 23 (shown in FIGS. 7 and 8)is installed as an aerodynamic fairing for the hub of proprotor 1413.Proprotor 1413 has a vibration attenuator 1401, comprising an adjustableweight assembly 1411, which is configured to be driven in rotation bystepper motor 1403. Weight assembly 1411 has at least one weight 1405that is movably attached to weight support 1407 for positioning alongtrack 1409 during operation of proprotor 1413. This configuration allowsfor weight 1405 to be selectively moved to any position between an innerradial position, which provides for minimal or no vertical forces asweight assembly 1411 spins, and an outer position, which provides formaximum vertical forces. A control system, such as control system 43 ofFIG. 1, is preferably provided for controlling the parameters ofoperation of vibration attenuator 1401, including positioning of weight1405, speed of rotation, direction of rotation, and phasing of thevertical forces relative to the position of the rotor.

In operation, control system 43 commands motor 1403 of vibrationattenuator 1401 to rotate weight assembly 1411 at a selected rotationalspeed and direction relative to standpipe 97, and control system 43 alsocommands weight 1405 to move to a selected position along track 1409 forproducing a selected amount of vertical force. In addition, controlsystem 43 will command motor 1403 to rotate weight assembly 1411 in amanner that produces a selected phasing of the vertical forces relativeto proprotor 1413.

Referring again to FIGS. 12-14, the vibration attenuators 1201 and 1401are coupled to the standpipe 97 and configured to produce verticalforces along the axis of the standpipe 97. The vertical forces aregenerated by rotating one or more unbalanced weights to create anoscillatory force to cancel rotor induced vibrations along the axis ofthe standpipe. For example, rotor induced vibrations that can becanceled with vibration attenuators 1201 and 1401 include lift forcevibrations that would otherwise travel down the rotor mast and into therotorcraft fuselage. Because vibration attenuators 1201 and 1401 aremounted on standpipe 97, while the rotor induced forces travel into mast95, bearing 101 can be configured so that the two sets of forces cancelin bearing 101. Alternately, the two sets of forces can cancel atadjoining structure near the base of the mast 95 and standpipe 97.

It should be appreciated that proprotors 1209 and 1301 may bealternatively configured to include one or more vertically orientedvibration attenuators 1201, without including attenuators 35 or 37.Similarly, proprotor 1413 may be alternatively configured to include oneor more vertically oriented vibration attenuators 1401, withoutincluding attenuators 130 and 105.

The vibration attenuator provides for several advantages, including: (1)improved capability of vibration attenuation; (2) attenuation ofvibration at the mast, instead of at the airframe; (3) improved controlof the vibration attenuators; (4) reduced weight; and (5) improvedreliability.

This description includes reference to illustrative embodiments, but itis not intended to be construed in a limiting sense. Variousmodifications and combinations of the illustrative embodiments, as wellas other embodiments, will be apparent to persons skilled in the artupon reference to the description. For example, embodiments of vibrationattenuators are shown installed on four-blade tiltrotor proprotors,though embodiments of vibration attenuators may be used on a tiltrotorproprotor having any number of blades and any other type of rotor, suchas a helicopter rotor or aircraft propeller. In addition, embodimentsare described herein as having stepper-type motors, though otherappropriate types of motors may be used.

The invention claimed is:
 1. A vibration attenuator for a rotor hub ofan aircraft, the vibration attenuator comprising: a weight adapted to bemounted to a non-rotating member of the rotor hub of the aircraft, eachweight also being adapted to be rotatable about a weight axis ofrotation which is perpendicular to a rotor mast axis, wherein the weightis selectively movable for changing a distance between a center of massof the weight and the weight axis of rotation; and a motor configured torotate the weight about the axis of rotation at a selected speed duringoperation; wherein during operation the weight is driven in rotation forcreating oscillatory vertical forces along the rotor mast axis forattenuation of rotor-induced vibrations having a selected frequency; andwherein the non-rotating member is stationary relative to an airframe ofthe aircraft.
 2. The vibration attenuator according to claim 1, whereinthe weight is generally disk-shaped and has a center of mass located aselected distance from the weight axis of rotation.
 3. The vibrationattenuator according to claim 1, wherein the non-rotating member is astandpipe.
 4. The vibration attenuator according to claim 1, wherein theweight comprises at least one set of two weights; and wherein theweights in each set are rotated about the weight axis of rotation in thesame direction during operation.
 5. The vibration attenuator accordingto claim 1, wherein the weight comprises at least one set of twoweights; and wherein during operation the weights of each set may berotated about the axis of rotation at a different rotational speed thanthe weights of another set, allowing attenuation of vibrations atmultiple frequencies.
 6. The vibration attenuator according to claim 1,wherein the weight comprises at least one set of two weights; andwherein during operation the weights of one set may be rotated about theweight axis of rotation in a direction different than the direction ofrotation of the weights of another set.
 7. The vibration attenuatoraccording to claim 1, wherein the weight comprises at least one set oftwo weights; and wherein during operation the weights of each set may beangularly positioned about the axis of rotation relative to each otherso as to produce no net force.
 8. The vibration attenuator according toclaim 1, wherein the motor is an electric motor.
 9. The vibrationattenuator according to claim 1, wherein the motor is adapted fortransferring torque to the weight for rotating the weight duringoperation.
 10. The vibration attenuator according to claim 1, whereinthe motor is coupled to the non-rotating member of the rotor hub with asupport.
 11. The vibration attenuator according to claim 1, wherein thenon-rotating member is supported in part by a bearing between thenon-rotating member and a rotating rotor mast.
 12. The vibrationattenuator according to claim 1, wherein the weight includes atranslatable weight that is configured to be selectively translated on atrack that extends radially from the weight axis of rotation.
 13. Thevibration attenuator according to claim 1, wherein the weight comprisesat least one set of two weights; and wherein the each set of weights maybe rotated about the weight axis of rotation in a manner that produces aselected phasing of the oscillatory vertical forces.
 14. A method ofattenuating vibrations in an aircraft having at least one rotor havingblades, the rotor having a rotor hub configured for being driven inrotation by a mast about a mast axis of rotation, the method comprising:(a) locating a rotatable weight in the rotor hub; (b) rotating eachweight at a selected speed about a weight rotation axis that isapproximately perpendicular to the mast axis of rotation, the weightbeing associated with a non-rotating member of the rotor hub, thenon-rotating member is stationary relative to the mast, and positioningthe weight for controlling a distance between a center of mass of theweight and the weight rotation axis; and (c) controlling the rotation ofthe rotatable weight for creating oscillatory vertical forces thatoppose rotor-induced vibrations having a selected frequency.
 15. Themethod according to claim 14, further comprising: (d) controlling therotation of the rotatable weight in manner that selectively phases theoscillatory vertical forces relative to the rotor hub.
 16. The methodaccording to claim 14, wherein step (b) comprises rotating the weight ata speed that is a multiple of the product of the number of blades of therotor multiplied by the rotational speed of the rotor.